Patterned SOI regions on semiconductor chips

ABSTRACT

A method and structure for forming patterned SOI regions and bulk regions is described wherein a silicon containing layer over an insulator may have a plurality of selected thickness&#39; and wherein bulk regions may be suitable to form DRAM&#39;s and SOI regions may be suitable to form merged logic such as CMOS. Ion implantation of oxygen is used to formed patterned buried oxide layers at selected depths and mask edges may be shaped to form stepped oxide regions from one depth to another. Trenches may be formed through buried oxide end regions to remove high concentrations of dislocations in single crystal silicon containing substrates. The invention overcomes the problem of forming DRAM with a storage capacitor formed with a deep, trench in bulk Si while forming merged logic regions on SOI.

This is a division of application Ser. No. 09/356,295, filed Jul. 16,1999, now U.S. Pat. No. 6,333,532.

FIELD OF THE INVENTION

This invention relates to Silicon-On-Insulator (SOI) semiconductor chipsand more particularly to patterned regions of SOI in bulk semiconductormaterial and further, to trenches formed at the perimeter of respectiveSOI regions to provide electrical isolation and to remove or controlcrystalline defects.

BACKGROUND OF THE INVENTION

It is well known that SOI based logic circuits show 20-30% higherperformance than logic circuits comparably made on bulk-Si. Currently,Si wafers are ion implanted with oxygen such as 10¹⁸ atoms/cm² to form aburied oxide region beneath the surface of the Si. The Si wafers areannealed to form a continuous buried oxide layer (BOX) beneath thesurface that isolates electrically the top Si layer from the bulk Sibelow the BOX. The above process for making SOI wafers is known in theart as separation by implantation of oxygen (SIMOX). SOI wafers are thenprocessed to form devices and/or circuits therein.

In the fabrication of CMOS circuits on bulk Si, shallow trench isolation(STI) has been used to provide electrical isolation between devices. Ashallow trench is formed, filled with an insulator and then planarizedby Chemical Mechanical Polishing (CMP).

In the fabrication of Dynamic Random Access Memories (DRAM), memorycells consisting of a field effect transistor and a capacitor have beenused. IBM Corp. has developed-the use of a deep trench capacitor for thememory cell such as described in U.S. Pat. No. 4,688,063 which issuedAug. 18, 1987 by Lu et al. entitled “Dynamic Ram Cell With MOS TrenchCapacitor In CMOS.” A deep trench is formed and then the sidewalls andbottom are oxidized or coated with an insulator followed by filling thetrench with a conductor such as doped poly silicon.

SUMMARY OF THE INVENTION

In accordance with the present invention, a structure for formingelectrical devices therein and a method for making is describedcomprising a semiconductor substrate containing Si having an uppersurface, and a plurality of spaced apart buried oxide regions formed byion implantation of oxygen therein through openings in a patterned maskto provide a plurality of buried oxide regions under the surface of asingle crystal silicon containing layer.

The invention further provides a method for forming a semiconductorlayer over an insulator comprising the steps of forming a first mask ona substrate containing Si, implanting oxygen through the mask into thesubstrate and annealing the substrate to form a patterned buried oxidelayer and a semiconductor layer there over.

The invention further provides a structure and method for forming bulksemiconductor regions and SOI regions on a wafer with trenchespositioned at or near the perimeter of the SOI regions to provideelectrical isolation and to control crystalline defect propagation andeffects.

The invention further provides a structure and method for formingembedded DRAM and merged logic by forming Bulk Si regions with DRAMformed therein and SOI regions with merged logic (CMOS) formed therein.

BRIEF DESCRIPTION OF THE DRAWING

These and other features, objects, and advantages of the presentinvention will become apparent upon consideration of the followingdetailed description of the invention when read in conjunction with thedrawing in which:

FIG. 1 shows a first embodiment of the invention with patterned buriedoxide regions under a thick silicon containing layer.

FIG. 2A shows a second embodiment of the invention with patterned buriedoxide regions under a thin silicon containing layer.

FIG. 2B shows FET's and a capacitor formed in the second embodiment ofFIG. 2A to form embedded DRAM and merged logic.

FIG. 3 shows a third embodiment of the invention with a plurality ofindependent thickness' of buried oxide regions.

FIGS. 4 and 5 show fourth and fifth embodiments of the invention withpatterned SOI regions with insulation there between.

FIG. 6 is a cross section view taken with Transmission ElectronMicroscopy (TEM) of the embodiment of FIG. 1 exposed to a high dose ofoxygen.

FIG. 7 is a cross section view taken with Transmission ElectronMicroscopy (TEM) of the embodiment of FIG. 2 exposed to a low dose ofoxygen.

FIG. 8 is a sixth embodiment of the invention showing trench structuresto replace crystalline defect regions and/or to block defects emanatingfrom regions having high dislocations or to control crystallographicdefects between SOI regions and bulk regions.

FIG. 9 shows a first decoupling capacitor in a bulk semiconductorsubstrate region.

FIG. 10 shows a second decoupling capacitor in a bulk semiconductorsubstrate region.

FIG. 11 shows a first body contact embodiment incorporating a trench.

FIG. 12 shows a second body contact embodiment incorporating a trench.

FIG. 13 shows field effect transistors formed on sidewalls of a trenchformed in a bulk semiconductor region.

FIG. 14 shows field effect transistors formed on sidewalls of a trenchhaving a buried oxide layer beneath the sidewalls.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawing, FIG. 1 shows an SOI structure 10 having asubstrate 12 of a poly or single crystalline semiconductor materialcontaining Si such as Si alone, SiGe, SiC, with a major upper surface13. On major surface 13, a dielectric mask 14 is formed. Dielectric mask14 may be of a material such as SiO₂, Si₃N₄, polysilicon,diamond-like-carbon, Al₂O₃ or combinations thereof. Dielectric mask 14is lithographically patterned to form openings 15 and 15′. Mask 14 maybe formed with one or more patterned dielectric layers.

A source of ions such as O⁺, O₂, O₃ and higher charge state ions 16 aredirected at surface 13 of substrate 12. Ions 16 are blocked fromentering substrate 12 where mask 14 remains and ions 16 pass throughopenings 15 and 15′ through exposed surface 13 into substrate 12 to formburied oxide regions 17 and 17′ and corresponding SOI regions 18 and 18′below openings 15 and 15′. The penetration depth of ions 16 is afunction of the ion energy from the ion source. The ion source may be anion implanter having an energy range from 60 to 210 keV which iscommercially available from IBIS Corporation, Danvers, Mass. The energyof ions 16 may be in the range from 30 keV to 10 MeV with specialequipment being fabricated to provide an ion source for either the highor low end of the energy range. Ions 16 may have insufficient energy topenetrate mask 14 to leave substrate 12 below mask 14 free of oxygenions 16 so as to be suitable for electronic devices. To prevent any ions16 from penetrating through mask 14, mask 14 should have a thicknesspreferably equal to the projected range (R_(p)) plus 6 times thestraggle distance (ΔR_(p)) of the gaussian distribution. In FIG. 1,buried oxide regions 17 and 17′ may have a thickness in range from 200 Åto 2 microns. The Si containing overlayer 29 above buried oxide regions17 and 17′ may have a thickness in the range from 100 Å to 4 microns.Buried oxide regions 17 and 17′ may be stoichiometric ornon-stoichiometric mixed with Si (crystalline or amorphous), SiO_(x), N,SiN_(x), C and SiC_(x).

The edge 20 of mask 14 is over edge 22 of buried oxide region or layer17. The edge 24 of mask 14 is over edge 11 of buried oxide region 17.The edge 19 of mask 14 is over edge 21 of buried oxide region 17′. Theedge 27 of mask 14 is over edge 32 of buried oxide region 17′. The edges20, 24, 19 and 27 of mask 14 and the respective edges 22, 11, 21 and 32of buried oxide regions 17 and 17′ may be offset with respect to oneanother to overlap or to be spaced apart. Buried oxide regions 17 and17′ may extend laterally underneath the edges of mask 14 which is normalin typical processing; or, buried oxide regions 17 and 17′ could befabricated so that edges 22, 11, 21 and 32 are spaced apart from edges20, 24, 19 and 27 respectively of mask 14. The shape of buried oxideedges 22, 11, 21 and 32 are a function of the physical shape of theedges of mask 14. Mask edge 24 is bevelled or slanted in the range from10 to 70 degrees with respect to an axis 23 which is orthogonal tosurface 13. Angled implantation of ions 16 at angles other than 90degrees may be in the range from 10 to 90 degrees with respect tosurface 13 which is used to move the edges 22, 11, 21 and 32 of theburied oxide layers 17 and 17′ away-from or underneath the edges 20, 24,19 and 27 of mask 14. Overlap distances or the spaced apart distances toedges 20 and 22, for example, are measured with respect to an orthogonalaxis 23 when passing through the respective edges being measured.

In FIG. 1, the silicon thickness along with the doping (p or n)) levelmay be adjusted so that the depletion region from source/drain junctionsdo not touch or interface with the upper surface 34 of buried oxidelayer 17 as shown in FIG. 2B.

In FIGS. 2A and 2B like references are used for functions correspondingto the apparatus of FIG. 1. In FIG. 2A, ions 16 of lower energy are usedto form buried oxide 17 at a shallower depth and hence a thinner Silayer 29 such as less than 1000 Å thick over buried oxide layer 17. InFIG. 2B, the silicon thickness along with the p or n doping level isadjusted to either allow the depletion region of source 25 and drain 26of Field Effect Transistor (FET) 28 to interface or not to interfacewith the upper surface 34 of buried oxide layer 17. The source 25 anddrain 26 itself may interface with upper surface 34 of buried oxidelayer 17 depending on the thickness of silicon containing over layer 29.

FIG. 2B shows a FET 28 having a source 25 and drain 26 formed in Sicontaining over layer 29 above buried oxide layer 17. An FET 30 having asource 31 and drain 33 is formed in Si containing substrate 12, a bulkSi region without buried oxide layer 17 below. FET's 28 and 30 may beformed concurrently or at separate times. Next to FET 30 is a trenchcapacitor 35 which may be formed coupled to FET 30 by way of conductorstrap 36 to form a memory cell 37 of an array of memory cells 43 in bulksemiconductor region 42. The structure of patterned SOI regions 18 withnon patterned bulk semiconductor regions 42 permits the formation ofembedded memory 43 in the form of RAM or DRAM in the bulk semiconductorregion 42 with logic circuitry formed from FET's such as CMOS formed inSOI regions 18 in Si containing over layer 29.

FIG. 3 shows a mask 36 which is adjusted in thickness to permit someions 16 to pass through mask 36 into substrate 12 to form a buried oxideregion 38 having a Si layer 39 above of a thickness less than 1000 Å andto form a buried oxide region 40 at a second depth where no mask or athinner mask (not shown) is intercepting ions 16 to form Si layer 41having a thickness greater than 1000 Å.

In a first case, the mask thickness of mask 36 is selected to determinethe depth of buried oxide 38 which would be less than the depth ofburied oxide 40 where no mask intercepts ions 16. A second case isdescribed where the same structure in FIG. 3 can be achieved by athicker mask 36 and with a higher energy of ions 16. The two buriedoxide regions 38 and 40 join together or are continuous from buriedoxide region 38 to buried oxide region 40 with a step 43 in buried oxidedepth between them corresponding to the location of the edge of mask 36above. The shape of step 43 is determined by the positions of buriedoxide regions 38 and 40 and by the shape of edge 44 of mask 36. It isunderstood that a plurality of thickness' of mask 36 may be used inconjunction with the energy of ions 16 to provide a silicon containingover layer with a plurality of thickness' in addition to over layer 39and 41.

FIG. 4 shows a mask 46 which is adjusted in thickness to permit someions 16 to pass through mask 46 into substrate 12 to form a buried oxideregion 48 extending to the surface 49 and to form a buried oxide region50 at a second depth where no mask or a thinner mask is interceptingions 16. The silicon thickness of Si layer 52 can be adjusted in amanner described in FIGS. 2A and 2B.

In a first case, the mask thickness of mask 46 is selected to determinethe depth of buried oxide 48 which will extend to the surface ofsubstrate 12. A second case is described where the same structure inFIG. 3 can be achieved by a thicker mask 46 and with a higher energy ofions 16. The two buried oxide regions 48 and 50 join together or arecontinuous from buried oxide region 48 to buried oxide region 50 with astep 53 in buried oxide depth between them. The shape of step 53 isdetermined by the positions of buried oxide regions 48 and 50 and by theshape of edge 47 of mask 46.

In FIG. 4, the silicon thickness of Si layer 52 along with the n or pdoping level is adjusted so that the depletion region from source/drainjunctions to be subsequently formed do or do not touch or intersectupper surface 51 of the buried oxide 50.

FIG. 5 shows a mask 56 which is adjusted in thickness to permit someions 16 to pass through mask 56 into substrate 12 to form a buried oxideregion 58 extending to the surface 59 of substrate 12 and to form aburied oxide region 60 at a second depth where no mask or a thinner maskis intercepting ions 16. The silicon thickness can be adjusted byadjusting the energy of ions 16 as described with respect to FIGS. 2Aand 2B.

In a first case, the mask thickness is selected to determine the depthof buried oxide 58 which will extend to the surface 59 of substrate 12.A second case is described where the same structure in FIG. 4 can beachieved by a thicker mask 56 and with a higher energy of ions 16. Thetwo buried oxide regions 58 and 60 join together or are continuous fromburied oxide region 58 to buried oxide region 60 with a step 63 inburied oxide depth between them. The shape of the step is determined bythe positions including the depth of buried oxide regions 58 and 60 andby the shape of mask edge 57 of mask 56.

In FIG. 5, the silicon thickness of Si layer 64 may be less than 1000 Åand is adjusted in thickness and doping to either allow the depletionregion from a subsequently formed source/drain region to touch, extendto, or intersect upper surface 61 of the buried oxide 60 or thesource/drain region itself contacts the upper surface 61 of buried oxide60.

FIG. 6 shows a TEM cross section micrograph of a patterned SOI structure10′ similar to FIG. 1 without mask 14 and with oxide layer 70 formed onsurface 13 of substrate 12 and SOI region 18. The thickness of buriedoxide 17 corresponds to the following implant conditions: oxygen energyabout 200 keV, oxygen dose about 1.8×10¹⁸ atoms cm⁻², implanttemperature about 570° C., and annealing at about 1320° C. for 6 hoursin nitrogen mixed with 2-3% oxygen. FIG. 6 shows a dense array ofdislocations 74 in substrate 12 at the edge of SOI region 18 anddislocations 78 at the end of Si containing layer 77 or near it whereinternal stresses are high in SOI region 18. The crystallinedislocations are above 1×10⁸ cm⁻², in substrate 12 and SOI region 18 atthe mask edge region of mask 14 (not shown) and at a physical dip ordepression in silicon surface 13. FIG. 6 shows oxide layer 70 aboveburied surface 13 grown during the step of annealing which may beremoved after the step of annealing by wet etching in dilutehydrofluoric (HF) acid.

FIG. 7 shows a TEM cross section micrograph of a patterned SOI structure10″ similar to FIG. 1 without mask 14. The thickness of buried oxidelayer 17 corresponds to the following implant conditions: oxygen energyabout 200 keV, oxygen dose about 2×10¹⁷ atoms cm⁻², implant temperatureabout 570° C., room temperature implant and annealing at about 1320° C.for 22 hours in argon mixed with at first times with 30-40% oxygen forabout 12 hours and then at second times with 2-3% oxygen for about 10hours. The steps of implanting at about 570° C., room temperatureimplanting and then annealing at about 1320° C. are described in Ser.No. 08/995,585 filed Dec. 22, 1997 (YO997117A allowed) by D. K. Sadanaand J. P. de Souza entitled “Defect Induced Buried Oxide (DIBOX) ForThroughput SOI” which is assigned to the assignee herein and isincorporated herein by reference. A method for forming SOI is alsodescribed in Ser. No. 09/034,445 filed Mar. 4, 1998 (YO996-264) by D. K.Sadana and O. W. Holland entitled “Method Of Forming Buried Oxide LayersIn Silicon” which is assigned to both the assignee herein and LockheedMartin Energy Research Corporation and which is incorporated herein byreference. FIG. 7 shows a few (three) dislocations 82 (about 1×10⁷ cm⁻²)in substrate 12 which is substantially better than that shown in FIG. 6at the mask edge region of mask 14 (not shown) and a dip in siliconsurface 13. Layer 86 is shown above buried surface 13 of substrate 12and is an oxide layer grown during the step of annealing which may beremoved after the step of annealing by wet etching in dilute HF acid.

FIG. 8, shows an embedded DRAM 90 and merged logic 92 separated by atrench 94. Trench 94 is positioned to remove crystalline defects such asdislocations 74 in substrate 12 and dislocations 78 in or at the edge ofSOI region 18 due to the stress during formation of buried oxide layer17 as shown in FIGS. 6 and 7. Trench 94 functions to remove crystallinedislocations and defects and thereby prevent dislocations and otherdefects from propagating into substrate 12 or into Si layer 29. In FIG.8, like references are used for functions corresponding to the apparatusof FIG. 2B. Trench 94 may be a deep trench in the range from 1 to 10microns formed by reactive ion etching (REI) or trench 94 may be ashallow trench having a depth in the range from 0.05 to 1 microns tointersect with buried oxide layer 17. Trench 94 may provide electricalisolation to layer 29 and between embedded DRAM 90 and merged logic 92.Merged logic 92 may have field effect transistors of both n and p typeinterconnected to form CMOS logic. FIG. 8 shows field effect transistor28 having source 25 and drain 26 in layer 29. Layer 29 may have athickness determined by the depth of buried oxide 17 and p or n dopingat a concentration wherein the depletion region in layer 29 extends ordoes not extend to the upper surface of buried oxide layer 17. By thedepletion region extending to the upper surface of buried oxide layer17, the capacitance of the drain and source are reduced. Where thedepletion region from source 25 and drain 26 in layer 29 from fieldeffect transistor 28 does not interface with buried oxide layer 17, thefield effect transistor operates in a partial depletion mode whicheliminates the kink effect in the I_(d) versus V_(d) graph at constantgate voltage due to the floating body effect i.e. charge accumulationbelow the channel.

In place of a single trench 94, a plurality of trenches 94 may be formedside by side parallel to one another such as where trench 35 is locatedto remove dislocations and to provide electrical isolation. Trench ortrenches 94 may have their respective sidewalls and bottom oxidized toform an insulator such as SiO₂ and filled with an oxide or apolysilicon. Selective placement of trenches 94 may also provide thermaldissipation from layer 29 or the top of substrate 12 to substrate 12below. To increase thermal conductivity, trench 94 may be filled with athermally conductive material such as a conductive oxide, metal or dopedpolysilicon.

Referring to FIG. 9, a decoupling capacitor 102 is shown formed insubstrate 12 in bulk semiconductor region 38 of substrate 12 which maybe adjacent one or more patterned SOI regions 18. A trench 104 is formedin upper surface 13 in substrate 12. The sidewalls 105 and bottom 106 oftrench 104 are covered by a layer of dielectric 108 such as silicondioxide, silicon nitride or combinations thereof. Trench 104 may befilled with a conductive material 109 such as p+ or n+ doped polysiliconor a metal. The top surface of conductive material 109 may be planarizedto be, for example, coplanar with surface 13 by chemical mechanicalprocessing (CMP).

Referring to FIG. 10, a decoupling capacitor 112 is shown formed insubstrate 12 in a bulk semiconductor region 18 of substrate 12 which maybe adjacent one or more patterned SOI regions 38. A trench 114 is formedin upper surface 13 in substrate 12. The sidewalls 115 and bottom 116 oftrench 114 and surface 13 are covered by a layer of dielectric 118 suchas silicon dioxide, silicon nitride or combinations thereof. Trench 114may be filled with a conductive material 119 such as p+ or n+ dopedpolysilicon or a metal. The top surface of conductive material 119 maybe planarized to be, for example, coplanar with the top surface ofdielectric 118 by a process such as CMP.

Referring to FIG. 11, a body contact 120 for field effect transistors 28shown in FIGS. 2B and 8 is shown by making electrical contact betweenlayer 29 and substrate 12 below buried oxide layer 17. A trench 122 isformed from surface 13 through layer 29, through buried oxide layer 17and into substrate 12. Trench 122 may have a dielectric layer 124 onsidewalls 125 from the bottom of sidewall 125 to the upper surface 126of buried oxide layer 17. Trench 122 may be filled with conductivematerial 119 such as a conductive oxide, p+ or n+ polysilicon or metal.

Referring to FIG. 12, a body contact 130 for field effect transistors 28shown in FIGS. 2B and 8 is shown by making electrical contact betweenlayer 29 and substrate 12 below buried oxide layer 17. A trench 132 isformed from surface 13 through layer 29, through buried oxide layer 17and into substrate 12 for a predetermined depth. Trench 132 hassidewalls 135. Trench 132 may be filled with a conductive material 129such as a conductive oxide, p+ or n+ polysilicon or metal.

Referring to FIG. 13, field effect transistors 142 and 144 are shownformed on sidewalls 143 and 145 respectively in trench 148. Field effecttransistor 142 comprises source 150, drain 151, gate dielectric 152 andgate electrode 154. Field effect transistor 144 comprises source 158,drain 159, gate dielectric 160, and gate electrode 161.

Referring to FIG. 14, field effect transistors 142 and 144 are shownformed on sidewalls 143 and 145 respectively in trench 148. Trench 148is formed such as by reactive ion etching (RIE) through the uppersurface 13 of substrate 12 which may contain silicon. Next, oxygen ions164 are implanted through surface 13 and into and beneath sidewalls 143and 145 of trench 148 by plasma immersion ion implantation to form aburied oxide layer 166. Plasma immersion ion implantation is anisotropic process for implanting oxygen ions shown by arrows 164 from aneffective non-directional source.

In FIGS. 1-14, like references are used for an apparatus correspondingto an apparatus in an earlier Figure.

While there has been described and illustrated a structure havingpatterned buried oxide regions and a process for making such aspatterned SOI semiconductor regions having one or more Si layerthickness'; patterned SOI semiconductor regions having SOI regions andbulk silicon containing regions where, for example, DRAM's may be formedin bulk regions and CMOS logic may be formed in SOI regions; andpatterned SOI regions having SOI and bulk silicon containing regionswith deep or shallow trenches therebetween, it will be apparent to thoseskilled in the art that modifications and variations are possiblewithout deviating from the broad scope of the invention which shall belimited solely by the scope of the claims appended hereto.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is:
 1. A method for forming a structurecomprising the steps of: forming a first mask having openings therein ona surface of a silicon containing substrate, said first mask furtherhaving first regions of a first thickness to permit ions of oxygen topass there through into said substrate with reduced energy, implantingoxygen through said openings and said first mask into said substrate,and annealing said substrate to form a plurality of first buried oxideregions below a silicon contain layer and a plurality of second buriedoxide regions in said substrate, whereby said second buried oxideregions are shallower than the first buried oxide regions.
 2. The methodof claim 1 wherein said step of implanting oxygen includes the step ofselecting an ion energy to form said silicon containing layer having athickness of less than 1000 Å.
 3. The method of claim 1 wherein saidstep of implanting oxygen includes the step of selecting an ion energyto form said silicon containing layer having a thickness of at least1000 Å.
 4. The method of claim 1 wherein said step of forming a firstmask includes the step of forming at least one of said first regionsadjacent a respective opening to form upon said step of annealing asecond buried oxide region contiguous with one of said plurality offirst buried oxide regions.
 5. The method of claim 1 further includingthe step of removing said first mask prior to said step of annealing. 6.The method of claim 1 wherein said step of implanting includes the stepof implanting at an angle with respect to the surface of said siliconcontaining substrate.
 7. The method of claim 6 wherein said step ofimplanting includes the step of implanting at an angle in the range from10 to 90 degrees.
 8. The method of claim 1 further including the step offorming a field effect transistor in said silicon containing layer. 9.The method of claim 1 further including the step of forming a trench insaid silicon containing substrate at the edge of at least one of saidsilicon-on-insulator regions.
 10. The method of claim 9 wherein saidstep of forming a trench includes the step of forming said trench tointersect an edge of said buried oxide region of saidsilicon-on-insulator region to remove a portion of the end of saidburied oxide.
 11. The method of claim 9 further including the step offilling said trench with a material selected from the group consistingof p+ polysilicon, n+ polysilicon and a metal.
 12. The method of claim 1further including the step of forming a trench in said siliconcontaining substrate through one of said silicon-on-insulator regions.13. A method for a structure comprising the steps of: forming a firstmask having openings therein on a surface of a silicon containingsubstrate, said first mask has a slanted edge at at least one of saidopenings at an angle with respect to an axis which is orthogonal to thesurface of the silicon containing substrate implanting oxygen throughsaid openings in said first mask and through said slanted edge into saidsubstrate, and annealing said substrate to form a plurality of firstburied oxide regions below a silicon contain layer, said first buriedoxide regions and said silicon containing layer are located in saidsubstrate, whereby said spaced apart silicon-on-insulator regions areformed and at least one of said first buried oxide regions has a shapededge.
 14. The method of claim 13 wherein said angle of said slanted edgeis in the range from 10 to 70 degrees.
 15. A method for forming astructure for forming semiconductor circuits comprising the steps of:selecting a semiconductor substrate containing silicon having aplurality of trenches therein; forming a first mask on said substratehaving an opening to expose a trench portion, and implanting oxygenthrough openings in said first mask into said substrate and said trenchportion, said step of implanting including the step of plasma immersionion implantation of oxygen whereby oxygen ions pass through thesidewalls of said trench portion to form a buried oxide layer withrespect to said sidewalls.
 16. The method of claim 15 further includingthe step of forming a gate dielectric on the sidewalls of said trench.17. The method of claim 16 further including the step of forming a gateelectrode on said gate dielectric on said sidewalls in said trench. 18.The method of claim 15 further including the step of covering a portionof at least one trench having sidewalls that extend to a common bottomwall with said first mask to prevent formation of a buried oxide layerwith respect to said sidewalls and common bottom wall of said at leastone trench.
 19. The method of claim 18 further including the step offorming a capacitor in said trench.
 20. The method of claim 18 furtherincluding the step of forming an electrical contact on the sidewalls andbottom of said at least one trench.